A comprehensive study of electrode compression effects in all vanadium redox flow batteries including locally resolved measurements

Graphite felts are the most commonly used electrode materials in vanadium redox flow batteries. In the conventional cell design, flat sheets of graphite bipolar plates and porous graphite felts are stacked without any bonding, which requires a certain degree of compression to minimize the contact resistance. Excessive compression of the electrode, however, leads to non-uniform flow distribution and potential occurrence of zones with the retarded flow of electrolyte. This study investigates a wide range of electrode compressions and their effect on the cell performance. The results show that a compression of 25% is the optimal trade-off between contact resistance, homogeneity of flow distribution and pumping losses. Moreover, spatially resolved measurements using a segmented cell are employed to visualize the flow distribution across the electrode in real time. The open circuit voltage after the termination of the cell charge/discharge is converted to the corresponding state of charge (SOC) of the electrolyte, and the difference between the theoretical and experimental state of charge of electrolyte is used to quantify the flow distribution across the electrode. The results show that the optimum conversion of the reactant can be achieved during a single pass at 25% electrode compression. This method of segmentation is simple and scalable to any size of the battery.Accepted versionThis research was financially and technically supported by Nanyang Technological University, Singapore, and SGL Carbon GmbH, Germany

Development of MEA materials for PEMFCs with a focus on performance and durability: Results from EU IMPACT and IMPALA projects

The market presence and public acceptance of fuel cells is slowly increasing, however, the mass market penetration is mainly hindered by the high costs, to which the precious metal content, in particular platinum, contributes significantly. Since most scenarios predict a further increase of platinum prices due to an increasing demand, it must be a major goal to reduce the amount of necessary noble precious metal catalysts in fuel cells. This presentation gives an overview of the combined efforts in the developments of ionomers and membranes (perfluorosulfonic acid and hydrocarbon based), catalysts (platinum and alloys), gas diffusion layers (GDL) and the resulting MEAs, that were and are performed in the EU funded projects ‘IMPACT’ and ‘IMPALA’. The aim is to reduce the precious metal loading to 0.2 mg/cm² in fuel cells with a power density of 1 W/cm² and a durability of min. 5000 h. The research addresses various issues, e.g. membrane conductivity, hydrophobicity, degradation mechanisms, catalyst supports. Substantial effort was made in order to increase the chemical and electrochemical stability of the catalysts in a wide range of operating conditions of temperature and humidity levels. For instance, carbon-supported PtNi and PtCo cathode electro-catalysts were developed to evaluate their performance and stability in PEFC cathodes under various conditions. The PtCo alloy catalyst, with a recorded mass activity >0.46 A/mg at 0.9 V RHE and 80°C with 50% RH is adequate to reach the MEA performance milestones. The membrane development comprise reinforced 22 µm and 10 µm thick AQUIVION® PFSA membranes that are used in the fabricated MEAs. Additionally novel hydrocarbon membranes are being tested. In parallel to membrane development, new PFSA dispersions with EW as low as 720 g/eq have been developed. The approach to improve the GDL performance and degradation involves sophisticated modelling to deeply analyze the water management inside the GDL and the simulation of its effective transport properties as a function of its local properties. The predictability of these models will be checked by comparison to some experiments planned in IMPALA. The final aim is to use modeling to have a better understanding of water management in MEA and propose improvements of GDL. A joint workshop of the projects IMPACT and IMPALA with more detailed presentations is held on Monday 2nd February in Toulouse directly at the FDFC site. The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) for Fuel Cell and Hydrogen Joint Technology Initiative under Grant No. 303452 (Impact)

Optimization of thermal oxidation of electrodes for the performance enhancement in all-vanadium redox flow betteries

Thermal oxidation of a graphite felt in air is the most widely used electrode activation method to enhance the performance of a vanadium redox flow battery. It has been reported extensively, but almost all previous studies used individual treatment protocols with only few parameter variations. In this paper, treatment time and temperature were varied systematically along with in-depth physicochemical and electrochemical analysis. Prolonged treatment time is chosen for oxidation at lower temperatures aiming at low material loss rates, whereas shorter periods are applied at higher treatment temperatures, which are associated with elevated material loss. Among the electrodes treated with various protocols, the best performance is observed with electrodes oxidized at 750 C for 5 min. This study is important for both, reproducibly preparing samples at lab scale as well as commercial developers, where optimized and cost-effective pretreatment plays a significant role.Accepted versionThis research was financially and technically supported by Nanyang Technological University, Singapore, and SGL Carbon, Germany

Novel Approaches for Solving the Capacity Fade Problem during Operation of a Vanadium Redox Flow Battery

The vanadium redox flow battery (VRFB) is one of the most mature and commercially available electrochemical technologies for large-scale energy storage applications. The VRFB has unique advantages, such as separation of power and energy capacity, long lifetime (>20 years), stable performance under deep discharge cycling, few safety issues and easy recyclability. Despite these benefits, practical VRFB operation suffers from electrolyte imbalance, which is primarily due to the transfer of water and vanadium ions through the ion-exchange membranes. This can cause a cumulative capacity loss if the electrolytes are not rebalanced. In commercial systems, periodic complete or partial remixing of electrolyte is performed using a by-pass line. However, frequent mixing impacts the usable energy and requires extra hardware. To address this problem, research has focused on developing new membranes with higher selectivity and minimal crossover. In contrast, this study presents two alternative concepts to minimize capacity fade that would be of great practical benefit and are easy to implement: (1) introducing a hydraulic shunt between the electrolyte tanks and (2) having stacks containing both anion and cation exchange membranes. It will be shown that the hydraulic shunt is effective in passively resolving the continuous capacity loss without detrimentally influencing the energy efficiency. Similarly, the combination of anion and cation exchange membranes reduced the net electrolyte flux, reducing capacity loss. Both approaches work efficiently and passively to reduce capacity fade during operation of a flow battery system

Investigation of Reactant Conversion in the Vanadium Redox Flow Battery Using Spatially Resolved State of Charge Mapping

Segmented cells enable real time visualization of the flow distribution in vanadium redox flow batteries by local current or voltage mapping. The lateral flow of current within thick porous electrodes, however, impairs the local resolution of the detected signals. In this study, the open circuit voltage immediately after the cessation of charge/discharge is used for the mapping of reactant conversion. This quantity is not hampered by lateral flow of current and can be conveniently transformed to the corresponding state of charge. The difference between theoretically calculated and experimentally determined conversion (change in the state of charge) across the electrode is used to determine local variations in conversion efficiency. The method is validated by systematic experiments using electrodes with different modifications, varying current densities and flow configurations. The procedure and the interpretation are simple and scalable to any size of flow cell

Vanadium redox flow battery with slotted porous electrodes and automatic rebalancing demonstrated on a 1 kW system level

Capacity loss due to electrolyte crossover through the membrane and pump losses due to pressure drop at the porous electrodes are widely known issues in vanadium redox flow batteries during operation. In commercial systems, these losses account for a significant reduction in the overall efficiency. Previous studies have been focused on the development of new membranes to solve the capacity loss, and design modification to reduce the pressure drop. In this work, we propose unique solutions to solve both problems and are demonstrated in a multi-cell stack for the first time. A 20-cell, 1 kW vanadium redox flow battery stack was assembled using thin bipolar plates and porous electrodes featuring interdigitated flow channels. Such a stack design is novel of its kind and can mitigate various problems associated with flow distribution and pump power in flow batteries. In addition, the electrolyte tanks were shunted together to rebalance the electrolyte automatically. The stack showed a very good and stable performance with an energy efficiency of 80.5% at a current density of 80 mA cm−2. The use of hydraulic shunt resulted in a constant capacity over 250 cycles while the use of flow channels on the porous electrodes resulted in ∼40% reduction in pressure drop, compared to a stack with standard felts. The reduction in pressure drop by employing flow channels reduced the pump power proportionally. Overall, capacity retention and utilization of active materials have been improved substantially. These methods are simple and applicable to any size of vanadium redox flow battery